To allow efficient combustion of gas, the gas is premixed with a controlled quantity and then passed into the hollow interior of a gas burner (40) whence it passes through the small apertures in the burner to a combustion chamber (38) where the mixture burns. The apertures in the gas burner are sized so no flashback from the combustion chamber occurs. The combustion chamber (38) is immersed in a tank (12) of liquid and the combustion products leave the combustion chamber (38) via an exhaust conduit (58, 60, 62, 64, 66, 68) which has a riser section (58) extending upwardly from the combustion chamber (38) and a helical section (60) descending from the top of the riser section (58). The exhaust conduit (58, 60, 62, 64, 66, 68) may be arranged so heat exchange can be effected between the combustion products and liquid being supplied to the tank (12) and/or liquid returning from a space heating system (82).